Re: [PATCH] KVM: s390: fix gisa destroy operation might lead to cpu stalls

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On 23.08.23 18:16, Claudio Imbrenda wrote:
On Wed, 23 Aug 2023 18:01:59 +0200
Alexander Gordeev <agordeev@xxxxxxxxxxxxx> wrote:

On Wed, Aug 23, 2023 at 04:09:26PM +0200, Michael Mueller wrote:

On 23.08.23 15:23, Alexander Gordeev wrote:
On Wed, Aug 23, 2023 at 02:41:40PM +0200, Michael Mueller wrote:
diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index 9bd0a873f3b1..73153bea6c24 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -3205,8 +3205,10 @@ void kvm_s390_gisa_destroy(struct kvm *kvm)
   	if (gi->alert.mask)
   		KVM_EVENT(3, "vm 0x%pK has unexpected iam 0x%02x",
   			  kvm, gi->alert.mask);
-	while (gisa_in_alert_list(gi->origin))
-		cpu_relax();
+	while (gisa_in_alert_list(gi->origin)) {
+		KVM_EVENT(3, "vm 0x%pK gisa in alert list during destroy", kvm);
+		process_gib_alert_list();

process_gib_alert_list() has two nested loops and neither of them
does cpu_relax(). I guess, those are needed instead of one you remove?

Calling function process_gib_alert_list() guarantees the gisa
is taken out of the alert list immediately and thus the potential
endless loop on gisa_in_alert_list() is solved. The issue surfaced
with the following patch that accidently disabled the GAL interrupt
processing on the host that normaly handles the alert list.
The patch has been reverted from devel and will be re-applied in v2.

88a096a7a460 Revert "s390/airq: remove lsi_mask from airq_struct"
a9d17c5d8813 s390/airq: remove lsi_mask from airq_struct

Does that make sense for you?

Not really. If process_gib_alert_list() does guarantee the removal,
then it should be a condition, not the loop.

this is actually a good question. why is it still a loop?

The reason for the loop aproach was that I was not sure if any late
incoming interrupts would bring the gisa back into the alert list
by the firmware.

I verified that this cannot happen if the mask that restores the
interruption alert mask (IAM) is properly set to 0x00 by the last
device driver de-registration before gisa destruction.

In addition I now enforce it to be 0x00 if not already done. (That would
be a bug.) That finally means the *if in_alert_list then process_alert_list* is sufficient.

I will send a v2.

But I am actually not into this code. Just wanted to point out that
cpu_relax() is removed from this loop and the two other loops within
process_gib_alert_list() do not have it either.

So up to Christian, Janosch and Claudio.

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